Hercules (Heracles), Fifth Labor : Cleaning the Augean Stables.

Labor number 5 was to clean the Augean stables. This was a particularly disgusting task given to Hercules by his cousin, King Eurystheus, to make good his sins. Hercules had succeeded in all his previous tasks and was beginning to earn the respect and loyalty of the ordinary people. This task was most probably set to humiliate or degrade him.
King Augeias was a very wealthy landowner, and had a massive stable with a very large number of stalls. He had a herd of cattle of about 3,000 head that he received as a divine gift. These cattle were immune to diseases, so cleaning out the stables for hygienic purposes was not on the list of things to do on any given day. Legend has it that the stables had not been cleaned for 30 years. A messy set up by all accounts.
Hercules duly arrived to do what had to be done, and negotiated a deal with Augeias that he would clean the stables within a day, for a 10th payment. He accepted, obviously thinking this could not be done.
So, Hercules went off and diverted the rivers Alpheius and Poneus through an opening he made in the stable wall. The water rushed through and all the accumulated waste drained out of a hole in the wall he had made on the other side. At the end of the day, the task was completed and the stables were cleaned.
At this point Augeias refused to pay, on account that this task had to be done without payment, as part of his debt to Eurystheus.
Other sources say that he refused to pay and Hercules summoned his son Phyleus to bear witness to his promise. He confirmed the deal was made and Phyleus was banished from the kingdom, and settled in Dulchium. Another version tells of a court case when his son testified against Augeias. In any event, Phyleus was banished and Hercules was a very angry man. He promised to return and exact vengeance at a later stage, which he did, and gave the Kingdom to his banished son.
The original labors were increased by one more, total 12, as this one, according to Eurystheus, did not count, as it had to be undertaken at no cost.
Image : Wikipedia